Because I knew when I joined a union shop that I would be paying dues to them in return for negotiating my compensation package. If I've trusted them enough with that, common sense would dictate that I trust they got the absolute best package they could for me. Now could the union sell me out and screw me? Certainly. But that's the risk I have decided to take.
I have been on this job for 27 years, have never been transferred far from home, have good salary and benefits, decent pension AND 401k with company match and job security- while other non union friends of mine have had multiple jobs over their lifetimes so far and have gone through hardships recently trying to find work. Others have done really well for themselves and make serious dough- but those are the guys who live at their jobs. I'm retiring at the end of this contract - all in all, the union has done a pretty decent job by me. I am an excellent worker who knows my job and takes it seriously- could I have done better somewhere else? Probably- but would have had a much harder time raising my kids, coaching their little league teams and being a scout leader for them is I was like some of my buddies who are afraid to ask for time off in fear of getting canned or demoted.